1. Field of the Invention
The present invention relates to a tracking control apparatus for an optical recording and reproducing apparatus for recording information data into a disk-shaped recording medium such as an optical disk, a magneto-optical disk or the like and for reproducing the recorded information data therefrom, and more particularly, to a tracking control apparatus for controlling a beam of light projecting from a light source such as a semiconductor laser onto a recording medium so that the beam of light scans on a track to be recorded or reproduced by correcting a tracking error signal according to an approximate equation of a function of a track address of the recording medium.
2. Description of the Related Art
Conventionally, there has been manufactured and publicly known, an optical recording and reproducing apparatus for focusing a beam of light modulated according to information data from a light source such as a semiconductor laser or the like, projecting the beam of light onto a disk-shaped recording medium rotating with a predetermined number of rotations per second, thereby recording the information data into the recording medium, and reproducing the recorded information data from the disk-shaped recording medium. In the disk-shaped recording medium of this type, there are formed minute tracks having a width of 0.6 .mu.m with a pitch of 1.5 .mu.m in the shape of a spiral or in concentric circles. In order to record an information data signal on the track of the recording medium or to reproduce the signal recorded on the track, a focus control is performed so that a beam of light is projected onto the recording medium in a predetermined focus state, and further, a tracking control is performed so that a beam of light scans precisely on the track.
Conventionally, the tracking control apparatus comprises:
(a) a photo-detector for detecting a reflected light from the recording medium or a transmission light which has transmitted through the recording medium, thereby generating a tracking error signal which changes depending on a positional relationship between a beam of light projected onto the recording medium and the track; PA1 (b) an actuator for moving an optical head including a focus lens for focusing and projecting a beam of light onto the recording medium in a direction approximately perpendicular to a tangent line of the track; and PA1 (c) a control circuit for driving the actuator based on the tracking error signal detected by the photodetector, and for performing a tracking control process so that a beam of light scans precisely on the track of the recording medium. PA1 light source means for generating and projecting a beam of light through an optical head onto a disk-shaped recording medium in which a plurality of tracks to be recorded or reproduced are formed in a shape of spiral circle or concentric circles; PA1 optical detecting means for detecting a reflected light from said recording medium; PA1 tracking error detecting means for detecting a positional shift of a beam of light projected onto said recording medium from a track to be recorded or reproduced based on said reflected light detected by said optical detecting means, and for generating a tracking error signal having a level of said detected positional shift; PA1 tracking control means for controlling said optical head so that said beam of light scans on said track to be recorded or reproduced based on said tracking error signal generated by said tracking error detecting means; PA1 offset detecting direct current means for detecting offset values of said tracking error signal on adjustment tracks predetermined among a plurality of tracks of said recording medium; PA1 first approximate function calculating means for calculating coefficients of a predetermined first approximate equation approximately representing an offset value characteristic on said track of said recording medium based on said offset values detected by said offset detecting means; PA1 offset value calculating means for calculating a direct current offset value corresponding to said track to be recorded or reproduced based on said first approximate equation with said calculated coefficients; and PA1 offset value correcting means for correcting an offset value of said tracking error signal based on said offset value calculated by said offset value calculating means. PA1 amplitude detecting means for detecting amplitude values of said tracking error signal on adjustment tracks predetermined among a plurality of tracks of said recording medium; PA1 second approximate function calculating means for calculating coefficients of a predetermined second approximate equation approximately representing an amplitude value characteristic on said track of said recording medium based on said amplitude values detected by said amplitude detecting means; PA1 amplitude value calculating means for calculating an amplitude value corresponding to said track to be recorded or reproduced based on said second approximate equation with said calculated coefficients; and PA1 amplitude value correcting means for correcting the amplitude of said tracking error signal based on said amplitude value calculated by said amplitude value calculating means. PA1 first jumping pulse generating means for sequentially generating and outputting to said tracking control means, a first pair of jumping pulses for jumping said track from an adjustment track to the next track of said adjustment track; PA1 first signal detecting means for sequentially detecting maximum and minimum levels of said tracking error signal generated by said tracking error detecting means, respectively, when said first jumping pulse generating means outputs said first pair of jumping pulses; PA1 second jumping pulse generating means for sequentially generating and outputting to said tracking control means, a second pair of jumping pulses for jumping said track from said next track to said adjustment track; PA1 second signal detecting means for sequentially detecting minimum and maximum levels of said tracking error signal generated by said tracking error detecting means, respectively, when said second jumping pulse generating means outputs said second pair of jumping pulses; and PA1 offset average calculating means for calculating an average value of an offset value of said tracking error signal on said adjustment track based on said maximum and minimum levels detected by said first signal detecting means and said minimum and maximum levels detected by said second signal detecting means, and for setting said calculated average value as said offset value. PA1 said offset average calculating means calculates an average value of the offset value of said tracking error signal on said adjustment track based on said outputted plurality of K sets of maximum and minimum levels detected by said first signal detecting means and said minimum and maximum levels detected by said second signal detecting means, and for setting said calculated average value as said offset value.
When an offset is generated in the tracking control apparatus due to, for example, a positional shift of the beam of light caused by an error in adjusting the optical head or stray light from the semiconductor laser or the like, a beam of light scans on a position slightly shifted from the center of the track. This leads to not only deterioration of the quality of the recording signal and the reproducing signal but also to lowering of the performance of pulling into a correct tracking control state since the above-mentioned tracking error signal is outputted asymmetrically with respect to a DC level thereof. In this case, a stable start and a stable retrieval can not be obtained in the tracking control apparatus.
Therefore, in order to establish a high reliability in the tracking control apparatus, it is necessary to adjust an offset of the tracking control apparatus. Conventionally, precise adjustment using a variable resistor for changing the offset value or the DC level of the tracking error signal requires much time in a process for manufacturing the recording and reproducing apparatus.
Recently, there has been suggested an automatic offset adjusting method for use in the tracking control apparatus of this type, using a microcomputer or a digital signal processor (DSP). The conventional automatic offset adjusting method includes a step of detecting an offset of the tracking control apparatus upon turning on the apparatus; and a step of correcting the offset so that the tracking control apparatus operates in an optimum tracking state.
For example, the automatic offset adjusting method of this type is described in Japanese Patent Laid-open Publication No. 1-128237. The published method includes a step of detecting an average value of a sinusoidal tracking error signal which is outputted when a beam of light crosses the track of the recording medium, and a step of applying a voltage over the tracking error signal so that the average value becomes a target value of the tracking control apparatus.
However, in the cases of adjusting the tracking control apparatus using the conventional methods, since an offset value of the tracking error signal is measured in only one region of the disk-shaped recording medium, when the offset value changes in the radial direction of the disk-shaped recording medium in the tracking control apparatus due to, for example, an inclination error caused when a direction in which a shaft moves the optical head is slightly inclined from a direction perpendicular to a tangent line of the track, or a disk tilt caused when a pair of disks are boned on each other or when a hubcap is mounted on the disk-shaped recording disk or the like, the effects of adjusting the tracking control apparatus are lowered.
Further, in a partial ROM optical disk comprising a ROM area and a re-writable RAM area which are formed so as to be divided in a radial direction of a disk-shaped magneto-optical disk, change rates in the amplitude of the tracking error signal change depending on the ROM and RAM areas due to the presence and non-presence of prepits. For example, when a beam of light is moved from the ROM area through a boundary area between the ROM and RAM areas to the RAM area and vice versa, the amplitude of the generated tracking error signal significantly changes depending on respective tracks of the ROM and RAM areas. Therefore, it is impossible to precisely detect the maximum and minimum values of the tracking error signal, and the preciseness of adjusting the tracking control apparatus is significantly lowered. In this case, a pulling-in operation into the tracking becomes unstable after retrieving a destination track to be recorded or reproduced, and the preciseness of retrieving the destination track becomes significantly lowered. Further, since the tracking control is performed so that a beam of light is slightly shifted from the center of a track, when a signal is recorded or reproduced in such a state, the quality of the recorded or reproduced signal becomes lowered, resulting in a low reliability of the tracking control apparatus.